1. Technical Field
The present invention relates generally to hydraulic lash adjusters and more particularly to a hydraulic lash adjuster which may be integrated into an engine over a wider range of operation angles.
2. Discussion
Hydraulic lash adjusters for automotive engines have been in use for many years, allowing engine manufacturers to eliminate lash between valve train components under varying operating conditions to maintain efficiency and to reduce noise in the valve train. Hydraulic lash adjusters operate on the principle of transmitting the energy of the valve actuating cam lobe through hydraulic fluid trapped in a pressure chamber underneath a plunger. As the length of the valve actuation components vary due to temperature changes, small quantities of hydraulic fluid are permitted to enter or escape from the pressure chamber during each operation of the cam lobe and thus effect an adjustment in the position of the plunger and consequently an adjustment in the lash of the valve train.
The cam lobe operating cycle comprises two distinct events: base circle and valve actuation. The base circle event is characterized by a constant radius between the center of rotation of the cam lobe and the cam follower during which effectively no cam energy is transmitted. The valve actuation event is characterized by a varying radius between the cam lobe center of rotation and the cam follower which effectively transmits cam energy to open an engine valve. During the valve actuation event, a portion of the load created by the valve spring, the inertia of valve train components and cylinder pressure is transmitted through the valve train including the lash adjuster, thus raising the pressure of the hydraulic fluid within the lash adjuster pressure chamber in proportion to the plunger area. As most modern hydraulic lash adjusters do not incorporate a seal between the adjuster body and plunger, the high pressure created by the load causes an amount of fluid to escape between the plunger and the inner wall of the lash adjuster body. As the fluid escapes from the pressure chamber, the plunger moves down in proportion to the amount of fluid which has escaped, causing the effective length of the lash adjuster to shorten. During the base circle event, the lash adjuster plunger spring moves the plunger up and eliminates the clearance between the valve actuation components. This in turn creates a pressure differential which causes hydraulic fluid to be drawn into the pressure chamber through the plunger check valve.
In modem hydraulic lash adjusters the escape of fluid from the pressure chamber is controlled by the clearance between the inner wall of the lash adjuster body and the plunger. As effective operation of the lash adjuster requires precise control of the amount of fluid escaping from the pressure chamber, the fit between the body and the plunger must be controlled very closely. To ensure the proper fit, it is typically necessary to selectively fit these components together. While this design has been received with commercial acceptance, several limitations are apparent.
One primary limitation concerns the applications in which these designs may be utilized. As these hydraulic lash adjusters are designed to leak, they must be incorporated into the engine within a relatively narrow range of operating angles to prevent fluid from draining out of the lash adjuster body and into the engine cavity when the engine is not operating and lost fluid is not being replenished. If this narrow range of installation angles is exceeded, fluid will drain out of the lash adjuster body into the engine cavity until the engine is operated and the fluid can be replenished. If the fluid in the lash adjuster drains to a very low level, the lash adjuster will not be able to eliminate all of the clearance in the valve train when the engine is subsequently started. This will cause air to be injected into the lash adjuster and significantly reduce its ability to remove the lash in the valve train due to the compressibility of the air. Consequently, the engine operator will notice a considerable level of noise caused by the excess clearance in the valve train. Therefore, there remains a need in the art for a hydraulic lash adjuster which can be utilized in a wider range of operating angles.